Perpendicular Spin-Transfer Torque (STT) Magnetic Random Access Memory (MRAM) is an embedded non-volatile memory technology based on magnetoresistance.
FIG. 1 illustrates a schematic diagram 100 of a perpendicular STT-MRAM stack 110 coupled to a transistor 120. Unlike typical RAM technologies, which store data as electric charge, MRAM data is stored by magnetoresistive elements. Generally, the magnetoresistive elements are made from two magnetic layers, each of which holds a magnetization. The magnetization of one layer (the “fixed layer” or “pinned layer” 110A) is fixed in its magnetic orientation, and the magnetization of the other layer (the “free layer” 110C) can be changed by a spin polarized programming current. Thus, the programming current can cause the magnetic orientations of the two magnetic layers to be either in a same direction, providing a lower electrical resistance across the layers (“0” state), or in opposing directions, providing a higher electrical resistance across the layers (“1” state). The switching of the magnetic orientation of the free layer 110C and the resulting high or low resistance states across the magnetic layers provide for the write and read operations of the typical MRAM cell.
The magnetic layer stack 110 in combination with top and bottom electrodes (not shown) located on top of the free layer and below the fixed layer, respectively, is known as a magnetic tunnel junction (MTJ). A programming current typically flows through the access transistor 120 and the MTJ. The fixed layer polarizes the electron spin of the programming current, and torque is created as the spin-polarized current passes through the MTJ. The spin-polarized electron current interacts with the free layer by exerting a torque on the free layer. When the torque of the spin-polarized electron current passing through the MTJ is greater than the critical switching current density, the torque exerted by the spin-polarized electron current is sufficient to switch the magnetization of the free layer. Thus, the magnetization of the free layer can be aligned to be either in a same or an opposing direction to the pinned layer, and the resistance state across the MTJ is changed.
FIG. 2 illustrates a schematic diagram of an STT-MRAM cell 200, which may be fabricated to form an array of memory cells in a grid pattern including a number of rows and columns, or in various other arrangements depending on the system requirements and fabrication technology. STT-MRAM cell 200 includes magnetic layer stack 210, bottom electrode 290, top electrode 295, bit line 220, source line 230, access transistor 240, word line 250, read/write circuitry 260, sense amplifier 270, and bit line reference 280.
A programming current is applied for the write operation of STT-MRAM cell 200. To initiate the programming current, read/write circuitry 260 may generate a write current to bit line 220 and source line 230. The polarity of the voltage between bit line 220 and source line 230 determines the switch in magnetization of the free layer in the magnetic layer stack 210. Once the free layer 210C is magnetized according to the spin polarity of the programming current, the programmed state is written to the STT-MRAM cell 200.
To read STT-MRAM cell 200, read/write circuitry 260 generates a read current to bit line 220 and source line 230 through magnetic layer stack 210 and transistor 240. The programmed state of STT-MRAM cell 200 depends on the resistance across magnetic layer stack 210 which may be determined by the voltage difference between bit line 220 and source line 230. In some embodiments, the voltage difference may be compared to a reference 280 and amplified by sense amplifier 270.
Vertical external magnetic fields can affect the magnetic moment of the free layer resulting in undesired bit flipping. It takes a vertical magnetic field of only a few hundred Oersted (Oe) to disturb stored information. Many applications of perpendicular STT-MRAMs require a higher magnetic robustness.
In-plane STT-MRAMs, as opposed to perpendicular STT-MRAMs, are susceptible to external magnetic fields that are horizontal. This is because in-plane STT-MRAMs are written by a programming current that causes the magnetic orientations of two magnetic layers to be either horizontally in parallel or antiparallel. Stability of in-plane MRAMs with respect to external magnetic fields can be increased using magnetic shields located on the top and bottom of the dies. However, this magnetic shielding concept is only applicable to in-plane STT-MRAMs, not perpendicular STT-MRAMs. For perpendicular STT-MRAM applications, it is the vertical magnetic fields that are of concern. Therefore, applications of perpendicular STT-MRAMs require effective shielding of vertical external magnetic fields.